This study will determine whether people with common variable immunodeficiency (CVID) with and without gastrointestinal (GI) symptoms have gut abnormalities (inflammation or loss of function) and changes in immune system cells and chemicals in the blood and gut. People with CVID have decreased levels of serum immunoglobulin IgG and IgA. Patients have sinus, lung and other infections, and many also have stomach and intestinal problems, such as chronic diarrhea, inability to absorb nutrition from food, and intestinal infections caused by bacteria.